The ability to draw three dimensional objects is a useful and important skill for product designers, artists, hobbyists, and other individuals. Drawing techniques that are used to accurately draw three dimensional objects are often not intuitive and generally require a significant amount of training and practice. Novices generally lack an understanding of the concepts of perspective and do not know how to measure and transfer proportions accurately. Consider an example of a wall that has a window directly in its center. If the wall is drawn from a perspective in which the left side of the wall is closer than the right side of the wall, the center of the wall will not be equidistant from the left and right sides of the wall in the drawing. Drawing the window at such a location would not provide an accurate drawing.
To ensure accurate proportions and perspective in their drawings, experienced artists learn various drawing techniques. Some coarse-to-fine drawing techniques, for example use scaffolds to ensure accurate proportions and perspective. A scaffold typically includes a preliminary sketch of basic geometric shapes (referred to herein as “primitives”) such as planes, cuboids, cylinders, and truncated pyramids that represent the general shapes of different parts of the object. Guidelines are used to ensure that the primitives are drawn with accurate perspective and proportion relative to one another. The contour lines of the actual parts of the object are drawn using the primitives for reference. The primitives of the scaffold can then be erased, leaving an accurate drawing of the object. In the wall/window example above, such a drawing technique can be used to ensure the window is drawn in an appropriate position relative to the wall and with accurate size and dimension. A plane-shaped primitive representing the general shape of the wall is first drawn. Diagonal guidelines connecting opposing corners of the primitive are then drawn and the center of the wall is identified using the intersection of the diagonal guidelines. A second primitive representing the general shape of the window is next drawn relative to the identified center of the wall. Finally, the details of the wall and window are drawn relative to the primitives and the primitives are erased. The resulting drawing accurately represents perspective and proportion.
While there are many books and videos that provide drawing tutorials, effective tutorials are difficult and time consuming to produce and often fail to offer tutorial content that is engaging to novices learning to draw. A tutorial author must carefully select an object and viewpoint, and then produce instructions for a sequence of steps that teach how to draw the object from the viewpoint. The tutorial is consequently limited to the particular object being drawn from the particular viewpoint. These limitations can reduce the appeal of the tutorial to novices learning to draw. It will often be more interesting for novices to learn by drawing objects that are of interest to them than by drawing the object in the tutorial. A novice interested in race cars will be less engaged by a tutorial for drawing a toaster than a tutorial for drawing a race car. Similarly, a novice may be more interested in drawing an object from his own selected viewpoint than using the viewpoint selected by the author of the tutorial.
Existing techniques that use computers to automatically generate tutorials allow experts to create tutorials for novices. However, such techniques generally fail to provide sufficient flexibility with respect to allowing a user to select an object and viewpoint for the tutorial. The types and characteristics of the objects used to automatically generate tutorials are limited. The techniques also often focus on stroke correction and beautification and do not provide adequate instructions for teaching novices to draw three dimensional objects with accurate proportions and perspective.